I know our house water supply is contaminated, both cold and hot, but I'd like to know how to hire a plumber competent to correct the problem. What questions can I ask to assure me that the plumber I'm talking to is up to the job?

We are on a municipal water system. The house is about 22 years old. The city says they can test our water, but they want me to taste the water from the hose bibb at the point where the water enters the house. Why do they want me to do this?